Sooo, all in all, I think this is a decent adaptation with an okayish casting, though the acting and scripting is a little lacking. The characters seem at times too grim, or lacking the humor that defined them.
Aang is a great choice as an actor, but the character is far too mature, instead of the kid he is supposed to be. Katara is more somber than in the animated version, and she and Aang lack chemistry. Originally I didn't like Zuko's actor choice (Zuko is my favorite character), but he grew on me by the end. Uncle Iroh is pretty good, but like could've been better? He seems too somber as well. I mean he's ok, but he doesn't inspire in you the ways the original uncle Iroh did, if that makes sense. I did not like the Azula, Mai, and Ty Lee castings at all. However Sokka is perfect. I think the actor is a tremendous choice that embodies Sokka both in looks and in character.
The plot is kinda messed up at times, with plot holes and some scenes that could've been done better. But overall I enjoyed watching the series and the special effects were amazing. But I felt that it lacked the epicness of the animated series.
I'm excited for seasons 2 and 3 though, to see where it goes.